This invention relates in general to respiratory devices and in particular to a new and useful evaluating device for liquid anesthetic and respiratory gas mixtures for patients.
In medical apparatus, where any malfunction may endanger the patient, it is a matter of course that the function of all parts and their assemblies is monitored by other, independently operating parts, or that interconnections are provided delivering signals as soon as differences between desired and actual values appear. Such signals release visual or acoustic alarms and/or automatic remedial measures, such as switching to standby assemblies.
A prior art apparatus for admixing liquid anesthetics to the breathing gas to be supplied to a patient is designed for monitoring the concentration, the breathing gas flow, and the amount of liquid anesthetic. The phenomenon is utilized that during the evaporation of a liquid anesthetic admixed to the respiratory gas, a temperature drop is observed which depends on the ratio between the amount of evaporated anesthetic and the amount of respiratory gas. Through an inlet, the respiratory gas along with the anesthetic are introduced from the outside into a thermally insulated mixing chamber. The supply conduit opens into the mixing chamber tangentially to form a cyclonic or whirling flow. Upstream of the mixing chamber, the supply conduit forms within the inlet a heat exchanger. Between the heat exchanger and the mixing chamber, an inlet temperature sensor extends into the inlet. An outlet for the mixture of respiratory gas and evaporated anesthetic connects to the outside, to the patient. Downstream of the mixing chamber, an outlet temperature sensor is provided. The wall of the mixing chamber is equipped with an electrical heating element. The two measuring sensors and the heating element are connected to a control and evaluating circuit which periodically switches between two operating phases. During the first phase, the heating is controlled to maintain equality between the temperature measurements of the two sensors. The supplied heating power thus corresponds to the heat of evaporation of the supplied amount of anesthetic, and is a measure of the flow of anesthetic. During the second phase, the heating is switched off and upon a complete evaporation, the temperature difference established between the two temperature sensors is determined. This difference depends on the ratio of the evaporative amount of anesthetic to the amount of respiratory gas, and thus is a measure of the concentration of the anesthetic in the respiratory gas. The quotient of the results of measurement in the two phases is a measure of the rate of flow of respiratory gas, i.e. EQU (flow of anesthetic)/(concentration of anesthetic)=flow of respiratory gas
(German Pat. No. 31 16 951, U.S. Pat. No. 4,477,395).